The field of the invention is sulfuric acid production processes and the invention relates more particularly to SO.sub.3 absorption towers, either inter-absorption towers or final absorption towers.
The classic concept of an SO.sub.3 absorption tower is shown in U.S. Pat. No. 1,013,638. A stream of sulfuric acid having a strength from 97% to 99.5% is fed onto an upper surface of a packing of quartz and drips downwardly through the packing. A stream of SO.sub.3 is caused to enter the base of the tower below the packing and move counter currently upwardly through the packing, thereby being absorbed by the sulfuric acid moving downwardly. This same process with many improvements continues today. In the Von Girsewald, et al. U.S. Pat. No. 2,071,598, concentrated sulfuric acid and SO.sub.3 are introduced into the top of a series of tubes and trickle downwardly as a counter current flow of a coolant is moved upwardly along the exterior of the tubes.
In the Detweiler U.S. Pat. No. 3,454,360 shows a basic single absorption sulfuric acid plant where acid is introduced into the top of an absorber and SO.sub.3 is introduced into the bottom.
The Dorr, et al. U.S. Pat. No. 4,368,183 utilizes a venturi where it is contacted with cocurrent sulfuric acid. The acid exiting the venturi is passed into a succeeding absorption stage through a packed layer and finally into a final counter current absorption tower.
The Cameron U.S. Pat. No. 4,654,205 shows an SO.sub.3 absorption process where acid is introduced into a mid-point of a packed bed and SO.sub.3 gas is passed upwardly from the bottom of the bed in a counter current manner.
The Peng U.S. Pat. No. 5,683,670 shows a sulfuric acid process which includes a counter current absorption tower.
The use of sulfuric acid continues to increase with increased population. The majority of sulfuric acid is used in the production of fertilizers for agricultural uses and as more and more fertilizers are required, more and more sulfuric acid is needed. The majority of sulfuric acid plants built today are large size plants producing between 1,000 and 2,000 tons per day. Typically, they are designed with three towers: A drying tower, an inter-absorption tower and a final absorption tower. The inter-absorption and final absorption towers are practically identical in construction. Both towers have heat exchangers, brink mist eliminators and candles installed in the upper section above the packing to remove essentially all acid mist particles. Typically, all three towers are packed with ceramic packing about 13' in depth. The packing beds are supported by ceramic bars resting on acid-proof brick piers. All sulfuric acid contact method processes start at the drying tower. Single absorption plants have only two towers, a drying tower and an absorption tower, and double contact double absorption plants have three towers, a drying tower, an inter-absorption tower and a final absorption tower. The drying tower has an input stream of air which is contacted with strong 98.5% sulfuric acid which is highly hygroscopic and removes essentially all moisture from the air. The moisture from the air tends to lower the strength of the circulating acid and to raise the acid level in the tower. Both tendencies are controlled by transferring acid to and from the drying tower so that the required acid strength as well as the required acid level are maintained as is the temperature. In the double contact double absorption process the first absorbing tower is an interpass tower which absorbs SO.sub.3 produced in a converter. The SO.sub.3 is absorbed in strong sulfuric acid (98.5%) to produce more sulfuric acid. In the absorption tower, the small amount of water (1.5%) present in the circulating acid causes the SO.sub.3 to react and become H.sub.2 SO.sub.4. SO.sub.2 is not absorbed in the circulating acid and is vented to the atmosphere in the case of a single absorption plant. In the case of a double absorption/double contact plant, the SO.sub.2 is returned to the converter after the first absorption tower to be converted to SO.sub.3 before the last pass which is performed through the final absorption tower. In this manner, most of the residual SO.sub.2 which wasn't oxidized to SO.sub.3 in the converter prior to the first absorption tower is now converted to SO.sub.3. The second and final charge of SO.sub.3 is then absorbed in the final absorption tower. The remaining gas, which is essentially SO.sub.3 free, is vented to the atmosphere. This vented gas still contains small amounts of SO.sub.2 which was not converted to SO.sub.3 in the converter and possible traces of SO.sub.3 which escape unconverted from the final acid tower. Environmental regulations are very strict and limit the amount of SO.sub.2 per ton of production that can be released to the atmosphere. This legal limitation obliges the producers to spend money for additional scrubbers or, alternatively, to try to improve the operation of the tower by using different packing materials, venturi scrubbers and the like, to comply with the law.
The problem of cleaning the exit gas is a serious one and is compounded by the increased demand for sulfuric acid. Most sulfuric acid plant production can be increased by 100-200 tons per day above the plant's original design capacity. Attempts to further increase production cause the conventional tower to be flooded which is unacceptable.
Inter-absorption and final absorption towers have a significant back pressure which is mainly caused by the counter current flow of the downwardly moving acid and the upwardly moving gas. This counter current flow also creates a significant amount of mist since the upwardly moving gas can carry the mist upwardly out of the packed bed. This creates the expensive use of large mist eliminators and the back pressure also consumes a great deal of energy. There is, thus, a need for a sulfuric acid process which reduces the amount of mist produced, and which reduces back pressure.